Known scotch yoke engine and or pump designs are known to include a piston assembly having a yoke structure with a shank to which is rigidly attached or pivotably mounted to or formed with a piston body of the piston assembly, the piston body including a crown and being adapted for reciprocation within a respective cylinder along the cylinder axis, the crown having a crown top surface and having a base or foot opposite the crown top surface. The base or foot includes a base surface. A shank of the yoke structure may be a protrusion or attached web member of the yoke structure extending generally towards the underside of the piston crown from the yoke structure proper and typically includes a surface of a kind and disposition suitable for mating as a member of a friction joint with the base surface and a fastener means is used to clamp the piston base surface and the shank surface together such that the piston base and the shank end face, surface and substantially immobile relative to each other as they are locked together with friction preventing generally any movement relative to each other. Others have clamped fast a coin-like washer between the piston base surface and the yoke structure shank surface and to align the piston with the yoke structure so that the piston and yoke structure are rigidly locked together it has been proposed in the prior art to equip the washer with a key system wherein there is provided a cruciform key having a midplate and wherein the arms of the cruciform are at 90 degrees to each other and laying on opposite sides of the washer midplate so that the mid plate lays between each grouping of respective key backs as it were. The yoke structure and the piston of the prior art each possessing complimentary key features and each aligned parallel to a respective key feature on the midplate of the crossed washer which is sandwiched between the piston base surface/s and the shank end surface/s and there being fixed into position by the action of a clamping means in the form of fastener arrangement extending between the piston and the yoke structure so that the washer midplate respective surface/s are engaged by and clamped onto by the piston base surface/s and the shank end face surface/s and held by friction thereby generated between the surfaces under the action of the constant clamping force of the fastener means when in engine operation such that crossed washer of the prior art remains immobile between the piston and yoke structure after fastener tightening of the piston onto the yoke structure, the prior art crossed washer and its key features laying dead and locked into position and therefore unable to move relative to either the piston or the yoke structure. Others have sought to attach a piston to a yoke structure by means of a gudgeon pin type arrangement and still others have sought to do away with the need to attach the piston to the yoke structure in the first place by way of making the yoke structure and the piston of one piece or as a unit.
Problems have arisen in executing a number of the objectives of the prior art designs. Attempts at clamping a piston to a yoke body in attempts to hold the rigidly together can cause the components to making up the joint to fret against each other or even friction weld together. The use of a gudgeon pin results in unwanted turning forces being introduced into the reciprocating piston assembly and piston noise and slap being generated thereby, the piston being able to turn around an axis being perpendicular relative to the piston axis, whilst the piston formed of a piece with the shank of the yoke can be costly to produce due to a need for a considerable up-scaling in alignment accuracy between certain of the various running surfaces of the reciprocating piston assembly.